The present invention is concerned with screen printing compositions and the use thereof, and is particularly concerned with screen printing compositions which find applicability for coating printed circuits and serve as solder masks.
In the fabrication of high quality printed circuit boards, it is necessary to coat the substrate with a tough, resistant coating in selected areas which serves as a solder mask. In high quality printed circuit boards, molten solder is used to insure good interconnection between the components and the circuitry.
The coating compositions to be used for such applications must possess a number of important characteristics. In particular, the coating compositions must be strongly adherent to a wide variety of metallic and nonmetallic substrates, and resistant to harsh environmental conditions such as high humidity and elevated temperatures. The compositions also must be impervious to a wide variety of solvents, resistant to a large number of different chemicals, and particularly be resistant to commonly-used flux compositions and solders.
Moreover, since it is preferred that the solder masks remain on the circuitry as a protective coating, such must possess electrical insulating properties which last over long periods of time. Also, the compositions must be durable and possess the above properties even after long periods of exposure to a wide variety of chemicals and adverse environmental conditions. The solder masks are left on the circuitry in order to protect it from oxidation and from the adverse effects of such gases as sulfur dioxide, hydrogen sulfide, and chlorine.
The compositions, if to be used in screen printing processes, must also be capable of good pattern definition which is dependent to a large extent upon the flow characteristics of the composition. Basically, the screen printing technique involves squeezing a coating composition through the open meshes of a stretched piece of material such as wire onto a printable substrate. The screen is covered or blocked out in part by a masking material in order to form the desired pattern on the printable substrate. The masking material may simply be a stencil or a dried lacquer, shellac or glue. Once the screen has been covered or blocked out in part by the masking material, it is held taut on a frame and positioned over the desired substrate. The coating composition is then poured onto the screen and squeezed through the open areas with a squeegee. Thereafter, the frame is removed and the coated substrate dried. It is important, when following a desired pattern, that the coating composition not flow or bleed outside of the preselected areas defined by the open areas of the screen but should follow accurately the image formed on the screen and reproduce it.
The present invention provides a screen printing composition which has the requisite properties to make it suitable as a solder mask and a permanent protective coating for high quality printed circuit boards. The screen printing compositions of the present invention not only possess the necessary physical characteristics and chemical resistivities required for such critical use, but also are extremely patternable in that the compositions will accurately reproduce the image on the printing screen. For instance, compositions within the scope of the present invention are capable of holding a pattern within about 4 mils or less.
In addition, the coating compositions of the present invention have improved pot life as compared to a number of other screenable compositions. The pot life of a composition is its stability after being mixed under normal ambient or room temperature conditions. Adequate pot life is essential in that sufficient time is needed after the composition is prepared for using the composition to coat the desired substrate without the composition degrading or curing. Also, the longer the pot life of a composition, the greater the quantity of it which can be prepared at any one time and be stored prior to use.
It has further been noted that the preferred compositions of the present invention greatly reduce the number of bubbles created in the coating during the curing process.